Detecting trace gases such as hydrogen chloride (HCl) in Mars' atmosphere is among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Terrestrially, HCl is closely associated with active volcanic activity, so its detection on Mars was expected to point to some form of active magmatism/outgassing. However, after its discovery using the mid-infrared channel of the TGO Atmospheric Chemistry Suite (ACS MIR), a clear seasonality was observed, beginning with a sudden increase in HCl

Martian gullies are landforms consisting of an erosional alcove, a channel, and a depositional apron. A significant proportion of Martian gullies at the mid-latitudes is active today. The seasonal sublimation of CO2 ice has been suggested as a driver behind present-day gully activity. However, due to a lack of in-situ observations, the actual processes causing the observed changes remain unresolved. Here, we present results from flume experiments in environmental chambers in which we created CO2-driven granular flows under Martian atmospheric conditions. Our experiments show that under Martian atmospheric pressure, large amounts of granular material can be fluidized by the sublimation of small quantities of CO2 ice in the granular mixture (only 0.5% of the volume fraction of the flow) under slope angles as low as 10°. Dimensionless scaling of the CO2-driven granular flows shows that they are dynamically similar to terrestrial two-phase granular flows, i.e. debris flows and pyroclastic flows. The similarity in flow dynamics explains the similarity in deposit morphology with levees and lobes, supporting the hypothesis that CO2-driven granular flows on Mars are not merely modifying older landforms, but they are actively forming them. This has far-reaching implications for the processes thought to have formed these gullies over time. For other planetary bodies in our solar system, our experimental results suggest that the existence of gully-like landforms is not necessarily evidence for flowing liquids but that they could also be formed or modified by sublimation-driven flow processes.

Subtle mounds have been discovered in the source areas of martian kilometer-sized flows and on top of summit areas of domes. These features have been suggested to be related to subsurface sediment mobilization, opening questions regarding their formation mechanisms. Previous studies hypothesized that they mark the position of feeder vents through which mud was brought to the surface. Two theories have been proposed: a) ascent of more viscous mud during the late stage of eruption and b) expansion of mud within the conduit due to the instability of water under martian conditions. Here we present experiments performed inside a low-pressure chamber, designed to investigate whether the volume of mud changes when exposed to a reduced atmospheric pressure. Depending on the mud viscosity, we observe volumetric increase of up to 30% at the martian average pressure of ~6 mbar. This is because the low pressure causes instability of the water within the mud, leading to the formation of bubbles that increase the volume of the mixture. This mechanism bears resemblance to the volumetric changes associated with the degassing of terrestrial lavas or mud volcano eruptions caused by a rapid pressure drop. We conclude that the mounds associated with putative martian sedimentary volcanoes might indeed be explained by volumetric changes of the mud. We also show that mud flows on Mars and elsewhere in the Solar System could behave differently to those found on Earth, because mud dynamics are affected by the formation of bubbles in response to the low atmospheric pressure.

Super-rotation affects - and is affected by - the distribution of dust in the martian atmosphere. We modelled this interaction during the 2018 global dust storm (GDS) of Mars Year 34 using data assimilation. Super-rotation increased by a factor of two at the peak of the GDS, as compared to the same period in the previous year which did not feature a GDS. A strong westerly jet formed in the tropical lower atmosphere, with strong easterlies above 60 km, as a result of momentum transport by thermal tides. Enhanced super-rotation is shown to have commenced 40 sols before the onset of the GDS, due to equatorward advection of dust from southern mid-latitudes. The uniform distribution of dust in the tropics resulted in a symmetric Hadley cell with a tropical upwelling branch that could efficiently transport dust vertically; this may have significantly contributed to the rapid expansion of the storm.

The vertical opacity structure of the martian atmosphere is important for understanding the distribution of ice (water and carbon dioxide) and dust. We present a new dataset of extinction opacity profiles from the NOMAD/UVIS spectrometer aboard the ExoMars Trace Gas Orbiter, covering one and a half Mars Years (MY) including the MY 34 Global Dust Storm and several regional dust storms. We discuss specific mesospheric cloud features and compare with existing literature and a Mars Global Climate Model (MGCM) run with data assimilation. Mesospheric opacity features, interpreted to be water ice, were present during the global and regional dust events and correlate with an elevated hygropause in the MGCM, providing further evidence for the role of regional dust storms in driving atmospheric escape as reported elsewhere. The season of the dust storms also had an apparent impact on the resulting lifetime of the cloud features, with events earlier in the dusty season correlating with longer-lasting mesospheric cloud layers. Mesospheric opacity features were also present during the dusty season even in the absence of regional dust storms, and interpreted to be water ice based on previous literature. The assimilated MGCM temperature structure agreed well with the UVIS opacities, but the MGCM opacity field struggled to reproduce mesospheric ice features, suggesting a need for further development of water ice parameterizations. The UVIS opacity dataset offers opportunities for further research into the vertical aerosol structure of the martian atmosphere, and for validation of how this is represented in numerical models.

Lower atmosphere variations in the martian water vapour and hydrogen abundance during the Mars Year (MY) 34 C storm from LS=326.1◦-333.5◦ and their associated effect on hydrogen escape are investigated using a multi-spacecraft assimilation of atmospheric retrievals into a Martian global circulation model. Elevation of the hygropause and associated increase in middle atmosphere hydrogen at the peak of the MY 34 C storm led to a hydrogen escape rate of around 1.4×109 cm−2s−1 , meaning the MY 34 C storm enhanced water loss rates on Mars to levels observed during global-scale dust storms. The water loss rate during the MY 34 C storm (a loss of 15% of the total annual water loss during only 5% of the year) was three times stronger than the weak MY 30 C storm assimilation, demonstrating that interannual variations in C storm strength must be considered when calculating the integrated loss of water on Mars.

We show a positive vertical correlation between ozone and water ice using a vertical cross-correlation analysis with observations from the ExoMars Trace Gas Orbiter’s NOMAD instrument. We find this is particularly apparent during the first half of Mars Year 35 (LS=0-180) at high southern latitudes, when the water vapour abundance is low. This contradicts the current understanding that ozone and water are, in general, anti-correlated. However, our simulations with gas-phase-only chemistry using a 1-D model show that ozone concentration is not influenced by water ice. Heterogeneous chemistry has been proposed as a mechanism to explain the underprediction of ozone in global climate models (GCMs) through the removal of HOX. We find improving the heterogeneous chemical scheme causes ozone abundance to increase when water ice is present, better matching observed trends. When water vapour abundance is high, there is no consistent vertical correlation between observed ozone and water ice and, in simulated scenarios, the heterogeneous chemistry does not have a large influence on ozone. HOX, which are by-products of water vapour, dominate ozone abundance and mask the effects of heterogeneous chemistry on ozone. This is consistent with gas-phase-only modelled ozone, showing good agreement with observations when water vapour is abundant. High water vapour abundance masks the effect of heterogeneous reactions on ozone abundance and makes adsorption of HOX have a negligible impact on ozone. Overall, the inclusion of heterogeneous chemistry improves the ozone vertical structure in regions of low water vapour abundance, which may partially explain GCM ozone deficits.

We present ~1.5 Mars Years (MY) of ozone vertical profiles, covering Ls = 163deg; in MY34 to Ls = 320deg; in MY35, a period which includes the 2018 global dust storm. Since April 2018, the Ultraviolet and Visible Spectrometer (UVIS) channel of the Nadir and Occultation for Mars Discovery (NOMAD) spectrometer aboard the ExoMars Trace Gas Orbiter has observed the vertical, latitudinal and seasonal distributions of ozone. Around perihelion, the relative abundance of ozone (and water from coincident NOMAD measurements) increases strongly together below ~40 km. Around aphelion, decreases in ozone abundance exist between 25-35 km coincident with the location of modelled peak water abundances. We report high latitude (above 55deg;), high altitude (40-55 km) equinoctial ozone enhancements in both hemispheres. The northern equinoctial high altitude enhancement is previously unobserved and forms prior to vernal equinox lasting for almost 100 sols (Ls ~350‑40deg), whereas the southern enhancement persists for over twice as long (Ls = ~5-140deg;). Both layers reform at autumnal equinox, with the northern layer at a lower abundance. These layers likely form through a combination of anti-correlation with water and the equinoctial meridional transport of O and H atoms to high-latitude regions. The descending branch of the main Hadley cell shapes the ozone distribution at Ls = 40-60deg;, with the possible signature of a northern hemisphere thermally indirect cell identifiable from Ls = 40-80deg;. The ozone retrievals presented here provide the most complete global description of Mars ozone vertical distributions as a function of season and latitude.

The local redistribution of granular material by sublimation of the southern seasonal CO2 ice deposit is one of the most active surface shaping processes on Mars today. This unique geomorphic mechanism has been linked to the dendritic, branching, ‘spider’-like araneiform terrain and associated fans and spots - features which are native to Mars and have no Earth analogues. However, there is a paucity of empirical data to test the validity of this genetic hypothesis. Additionally, it is unclear whether the organised radial patterns of araneiforms require a singular or multiple seasonal events to form. Here we present the results of a suite of laboratory experiments undertaken to investigate if the interaction between a sublimating CO2 ice overburden with central vents and a porous, mobile regolith will mobilise grains from beneath the ice in the form of a plume to generate araneiform patterns. We investigate the physical constraints on the level of branching of these features and the area that they cover. We provide the first observations of plume activity via CO2 sublimation and consequent erosion of araneiform features. Based on calculations of the mass flux sublimation rate of CO2 ice blocks buried under sediment and consequent volume transport, we show that CO2 sublimation can be a highly efficient agent of sediment transport under Martian pressure.

Solar occultations performed by the Nadir and Occultation for MArs Discovery (NOMAD) ultraviolet and visible spectrometer (UVIS) onboard the ExoMars Trace Gas Orbiter (TGO) have provided a comprehensive mapping of ozone density, describing the seasonal and spatial distribution of atmospheric ozone in detail. The observations presented here extend over a full Mars year between April 2018 at the beginning of the TGO science operations during late northern summer on Mars (Ls = 163°) and March 2020. UVIS provided transmittance spectra of the martian atmosphere in the 200 - 650 nm wavelength range, allowing measurements of the vertical distribution of the ozone density using its Hartley absorption band (200 – 300 nm). Our findings indicate the presence of (1) a high-altitude peak of ozone between 40 and 60 km in altitude over the north polar latitudes for over 45 % of the martian year, particularly during mid-northern spring, late northern summer-early southern spring, and late southern summer, and (2) a second, but more prominent, high-altitude ozone peak in the south polar latitudes, lasting for over 60 % of the year including the southern autumn and winter seasons. When they are present, both high-altitude peaks are observed in the sunrise and sunset occultations, indicating that the layers could persist during the day. Model results from the GEM-Mars General Circulation predicts the general behavior of the high-altitude peaks of ozone observed by UVIS and are used in an attempt to further our understanding of the chemical processes controlling the high-altitude ozone on Mars.